In wireless communication systems, diversity has been an effective technique in combating detrimental effects of channel fading caused by multi-path propagation and Doppler shift. Specifically, a spatial diversity can be achieved by a multiple-input-multiple-output (MIMO) system by exploiting space time coding. However, in a cellular network or wireless sensor networks, due to the limited size, cost and hardware limitation, it may not be possible for a mobile terminal or wireless sensor nodes to equip with multiple transmit antennas.
To overcome such limitation, a new form of diversity technique, called user cooperation diversity or distributed spatial diversity, has been proposed recently, for cooperative cellular networks or wireless sensor networks. The idea is to allow different users to communicate cooperatively and share their antennas to achieve a virtual MIMO link, thus achieving a spatial diversity gain. The relayed transmission can be viewed as a good example of distributed diversity techniques. In the relay channel, the source sends the source information to the relays. The relays detect the received signals, reconstruct them and then forward them to the destination. At the destination, by properly combining the received signals from the source and relays, a distributed diversity can be achieved.